Electrode structure for solid-polymer type fuel cell

ABSTRACT

An electrode structure for a polymer electrolyte fuel cell comprises a pair of electrode catalyst layers and a polymer electrolyte membrane held between the electrode catalyst layers. The polymer electrolyte membrane is a sulfonation product of a polymer which comprises a main chain wherein two or more divalent aromatic residues are bound to one another directly or through oxy groups or divalent groups other than aromatic residues and side chains comprising aromatic groups to be sulfonated. The number of divalent aromatic residues comprised in the main chain of the above polymer is denoted by X, and the number of oxy groups comprised in the main chain of the above polymer is denoted by Y, and the value X/Y is within the range of from 2.0 to 9.0.

TECHNICAL FIELD

The present invention relates to an electrode structure used for apolymer electrolyte fuel cell.

BACKGROUND ART

The petroleum source is beginning to exhausted, and at the same time,environmental problems such as global warming due to the consumption offossil fuel have increasingly become serious. Thus, a fuel cell receivesattention as a clean power source for electric motors that is notaccompanied with the generation of carbon dioxide. The above fuel cellhas been widely developed, and some fuel cells have become commerciallypractical. When the above fuel cell is mounted in vehicles and the like,a polymer electrolyte fuel cell comprising a polymer electrolytemembrane is preferably used because it easily provides a high voltageand a large electric current.

As an electrode structure used for the above polymer electrolyte fuelcell, there has been known an electrode structure, which comprises apair of electrode catalyst layers comprising a catalyst such as platinumsupported by a catalyst carrier such as carbon black that is formed byintegrating by an ion conducting polymer binder, a polymer electrolytemembrane capable of conducting ions sandwiched between the electrodecatalyst layers, and a backing layer laminated on each of the electrodecatalyst layers. When a separator acting also as a gas passage isfurther laminated on each of the electrode catalyst layers, the aboveelectrode structure constitutes a polymer electrolyte fuel cell.

In the above polymer electrolyte fuel cell, one electrode catalyst layeris defined as a fuel electrode, and the other electrode catalyst layeris defined as an oxygen electrode. Now, reducing gas such as hydrogen ormethanol is introduced into the fuel electrode through the above backinglayer, whereas oxidizing gas such as air or oxygen is introduced intothe oxygen electrode through the above backing layer. By this action, onthe above fuel electrode side, protons are generated from the abovereducing gas by the action of a catalyst contained in the aboveelectrode catalyst layer. Then, the protons transfer to the electrodecatalyst layer on the above oxygen electrode side through the abovepolymer electrolyte membrane. Thereafter, the protons are reacted withthe above oxidizing gas introduced into the oxygen electrode by theaction of the above catalyst contained in the electrode catalyst layeron the above oxygen electrode side, so as to generate water. Thus, theabove fuel electrode is connected to the above oxygen electrode throughusing a conductor, so as to obtain electric current.

Previously, in the above electrode structures, a perfluoroalkylenesulfonic acid polymer (e.g., Nafion (trade name) from DuPont) has beenwidely used for the above polymer electrolyte membrane. Theperfluoroalkylene sulfonic acid polymer is sulfonated, and accordinglyit has an excellent proton conductivity. The compound also has achemical resistance as a fluorocarbon resin. However, the compound has aproblem in that it is extremely expensive.

Thus, the use of a relatively inexpensive ion conducting materialinstead of the perfluoroalkylene sulfonic acid polymer has been understudy for constituting an electrode structure for a polymer electrolytefuel cell. An example of the above inexpensive ion conducting materialmay include a hydrocarbon-based polymer.

However, the hydrocarbon-based polymer is poor in toughness, and so itis difficult to use it as a polymer electrolyte membrane to constitutethe above electrode structure. In order to improve the toughness of thehydrocarbon-based polymer, for example, methods such as introducing abending group into the main chain of the hydrocarbon-based polymer, orreducing the ion exchange capacity are being considered.

However, when the hydrocarbon-based polymer whose toughness is improvedas described above is used for the polymer electrolyte membrane of theelectrode structure, there is an inconvenience in that it is difficultto obtain a sufficient power generation efficiency. In addition, thehydrocarbon-based polymer is inconvenient in that it has a low oxidationresistance and it deteriorates rapidly.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to solve such inconvenience andto provide an electrode structure for a polymer electrolyte fuel cell,which comprises a polymer electrolyte membrane having an excellenttoughness and has an excellent power generation efficiency.

Moreover, it is another object of the present invention to provide anelectrode structure for a polymer electrolyte fuel cell having anexcellent oxidation resistance and an excellent power generationefficiency.

Furthermore, it is another object of the present invention to provide apolymer electrolyte fuel cell having an excellent power generationefficiency.

To eliminate the above inconvenience, the electrode structure for apolymer electrolyte fuel cell of the present invention comprises a pairof electrode catalyst layers and a polymer electrolyte membranesandwiched between both the electrode catalyst layers, characterized inthat the above polymer electrolyte membrane is a sulfonation product ofa polymer, comprising a main chain, in which two or more divalentaromatic residues are bound to one another directly or through oxygroups or divalent groups other than aromatic residues, and side chainscomprising aromatic groups to be sulfonated.

As a result of various studies regarding the above describedhydrocarbon-based polymer constituting a polymer electrolyte membrane,the present inventors have found that a polymer electrolyte membranehaving an excellent toughness can be obtained by setting the ratiobetween the number of divalent aromatic residues constituting the mainchain of the above polymer and the number of oxy groups binding to theabove aromatic residues within a specific range.

Thus, in the first aspect, the electrode structure for a polymerelectrolyte fuel cell of the present invention is characterized in that,provided that the number of divalent aromatic residues comprised in themain chain of the above polymer is denoted by X, and the number of oxygroups comprised in the same above main chain is denoted by Y, the valueX/Y is within the range between 2.0 and 9.0.

In the first aspect of the present invention, when the value X/Y that isthe ratio between the unit number X of divalent aromatic residuescomprised in the main chain of the above polymer and the unit number Yof oxy groups comprised in the same above main chain is within the rangebetween 2.0 and 9.0, the above polymer electrolyte membrane can beexcellent in toughness and ion conductivity. As a result, an electrodestructure can be easily produced using the above polymer electrolytemembrane, and further, the obtained electrode structure can have anexcellent power generation efficiency.

If the above X/Y is less than 2.0, the above polymer electrolytemembrane cannot obtain a sufficient ion conductivity. If the X/Y exceeds9.0, it cannot obtain a sufficient toughness.

Moreover, as a result of various studies regarding the above describedhydrocarbon-based polymer constituting a polymer electrolyte membrane,the present inventors have found that the level of hydrophobicity of theabove hydrocarbon-based polymer can be expressed by a function that usesthe number of groups containing an aromatic group to be sulfonated in aside chain thereof, the number of divalent aromatic residues that cannotbe sulfonated, and the number of oxy groups with respect to the totalgroups comprised in the main chain of the above polymer, and that ahydrocarbon-based polymer having an excellent oxidation resistance canbe obtained by setting the above level of hydrophobicity within acertain range. They have also found that a polymer electrolyte membranehaving an excellent power generation efficiency can be obtained bysulfonating the above hydrocarbon-based polymer whose hydrophobic levelis within a certain range and thereby imparting a certain ion exchangecapacity.

Thus, in the second aspect, the electrode structure for a polymerelectrolyte fuel cell of the present invention is characterized in that,provided that the number of groups to be sulfonated is denoted by A, thenumber of nonsulfonated divalent aromatic residues is denoted by B, andthe number of oxy groups is denoted by C with respect to the totalgroups comprised in the main chain of the above polymer, the value(B/C)×(B+C)−A is within the range between 35 and 380.

In the second aspect of the present invention, the hydrophobic level ofthe above polymer is represented as the difference between thehydrophilic level and the hydrophobic level of the above polymer. Thehydrophilic level is herein represented by the number of groups to besulfonated A with respect to the total groups contained in the mainchain of the above polymer.

On the other hand, the hydrophobic level relates to the number ofnonsulfonated divalent aromatic residues B and the number of oxy groupsC with respect to the total groups contained in the main chain of theabove polymer. As the ratio B/C of the number of nonsulfonated divalentaromatic residues B to the number of oxy groups C becomes great and thesum of both numbers B+C also becomes great, the hydrophobic levelbecomes high. Hence, the above hydrophobic level is represented by(B/C)×(B+C).

As a result, the hydrophobic level of the above polymer is representedby formula (I) indicated below. It should be noted that, in the presentdescription, hereinafter the above “hydrophobic level” is referred to asa “hydrophobic index.”Hydrophobic index=(B/C)×(B+C)−A  (I)

In the second aspect of the present invention, when an electrodestructure for a polymer electrolyte fuel cell comprises a polymerelectrolyte membrane obtained by sulfonating the above polymer having ahydrophobic index within the range between 35 and 380, the electrodestructure becomes excellent in oxidation resistance and power generationefficiency. If the hydrophobic index is less than 35 or more than 380, asufficient oxidation resistance cannot be obtained.

In each of the above aspects of the present invention, the main chain ofthe above polymer comprises a first repeating unit represented by thefollowing general formula (1) and a second repeating unit represented bythe following general formula (2), and it may further comprise a thirdrepeating unit represented by the following general formula (3):

wherein A represents an electron attracting group, B represents anelectron releasing group, n is an integer of 0 or 1, and a benzene ringincludes a derivative thereof,

wherein A represents an electron attracting group, Y represents—C(CF₃)₂— or —SO₂—, and a benzene ring includes a derivative thereof,and

wherein B represents an electron releasing group.

It should be noted that the term “electron attracting group” is used inthe present description to mean a divalent group such as —CO—, —CONH—,—(CF₂)p- (wherein p is an integer of 1 to 0), —C(CF₃)₂—, —COO—, —SO— or—SO₂—, in which the Hammett substituent constant is 0.06 or greater inthe meta position of a phenyl group and it is 0.01 or greater in thepara position thereof. It should be also noted that the term “electronreleasing group” is used herein to mean a divalent group such as —O—,—S—, —CH═CH—, or —C≡C—.

Herein; sulfonation occurs only to a benzene ring to which no electronattracting group binds. Accordingly, when a polymer of which main chaincomprises the first repeating unit represented by general formula (1)and the second repeating unit represented by general formula (2) issulfonated, no sulfonic acid group is introduced either onto any benzenering of the first repeating unit, which belongs to the main chain or anybenzene ring of the second repeating unit, but it is only introducedonto benzene rings belonging to the side chain of the first repeatingunit. Thus, in the above polymer, the molar ratio between the firstrepeating unit and the second repeating unit is adjusted to control theamount of the introduced sulfonic acid groups, so that the ionconductivity of a polymer electrolyte membrane can be adjusted.

In addition, the main chain of the above polymer comprises the thirdrepeating unit represented by general formula (3) as well as the firstrepeating unit represented by general formula (1) and the secondrepeating unit represented by general formula (2), so that, in the firstaspect, the electrode structure can adopt a structure which imparts abending ability to the polymer without introducing a sulfonic acidgroup, while controlling the number of oxy groups. Otherwise, in thesecond aspect, while controlling the number of oxy groups C, the numberof nonsulfonated divalent aromatic residues B is allowed to increase, sothat the hydrophobic index can be controlled.

The electrode structure in each of the above aspects of the presentinvention constitutes a polymer electrode fuel cell, which generatespower, when oxidizing gas is supplied to one side of the above electrodestructure and reducing gas to the other side.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative sectional view of the electrode structure ofthe present invention;

FIG. 2 is a graph showing the relationship between the value X/Y and thetoughness of the polymer electrolyte membrane;

FIG. 3 is a graph showing the relationship between the value X/Y and theion conductivity of the polymer electrolyte membrane; and

FIG. 4 is a graph showing the relationship between the hydrophobic indexand the oxidation resistance of the polymer electrolyte membrane.

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1, the electrode structure of a first embodiment of thepresent invention comprises a pair of electrode catalyst layers 1, 1, apolymer electrolyte membrane 2 sandwiched between both the electrodecatalyst layers 1, 1, and backing layers 3, 3 laminated on the electrodecatalyst layers 1, 1 respectively.

The electrode catalyst layer 1 is produced by screen printing a catalystpaste consisting of a catalyst particle and an ion conducting polymerbinder on the backing layer 3, so that a certain amount (e.g., 0.5mg/cm²) of catalyst is kept thereon, and then drying it. The abovecatalyst particle consists of a platinum particle that is supported bycarbon black (furnace black) at a certain weight ratio (e.g., carbonblack:platinum=1:1). The above catalyst paste is prepared by uniformlydispersing the above catalyst particles in a solution containing an ionconducting polymer binder such as a perfluoroalkylene sulfonic acidpolymer (e.g., Nafion (trade name) from DuPont) at a certain weightratio (e.g., catalyst particle:binder solution=1:1).

The backing layer 3 consists of a substrate layer and a carbon paper.The above substrate layer is formed by mixing carbon black andpolytetrafluoroethylene (PTFE) particles at a certain weight ratio(e.g., carbon black:PTFE particle=4:6), uniformly dispersing theobtained mixture in a solvent such as ethylene glycol so as to obtain aslurry, and applying the slurry on the one side of the above carbonpaper followed by drying it. The catalyst paste screen printed on thebacking layer 3 is dried, for example, by drying at 60° C. for 10minutes and then vacuum drying at 120° C. for 60 minutes.

The polymer electrolyte membrane 2 is a sulfonate of a copolymerobtained by polymerizing a first repeating unit represented by generalformula (1) indicated below and a second repeating unit represented bygeneral formula (2) indicated below at a predetermined molar ratio.Alternatively, the polymer electrolyte membrane 2 is a sulfonationproduct a copolymer obtained by polymerizing the first repeating unitrepresented by the same general formula (1) indicated below, the secondrepeating unit represented by the same general formula (2) indicatedbelow, and a third repeating unit represented by general formula (3)indicated below at a predetermined molar ratio:

wherein A represents an electron attracting group, B represents anelectron releasing group, n is an integer of 0 or 1, and a benzene ringincludes a derivative thereof,

wherein A represents an electron attracting group, Y represents—C(CF₃)₂— or —SO₂—, and a benzene ring includes a derivative thereof,and

wherein B represents an electron releasing group.

An example of a monomer used as the first repeating unit represented bythe above general formula (1) includes2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone represented by thefollowing formula (4).

Examples of a monomer used as the second repeating unit represented bythe above general formula (2) include2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropanerepresented by the following formula (5) and2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]sulfone represented by thefollowing formula (6):

An example of a monomer used as the second repeating unit represented bythe above general formula (3) may include 4,4′-dichlorobenzophenone.

The above copolymer preferably has a polymer molecular weight of 10,000to 1,000,000 at a weight-average molecular weight shown usingpolystyrene conversion. If the above polymer molecular weight is lessthan 10,000, a mechanical strength that is preferable as a polymerelectrolyte membrane might not be obtained. If it exceeds 1,000,000, asdescribed later, when the polymer is dissolved in a solvent to form amembrane, the dissolubility decreases or the viscosity of the solutionincreases, and thereby it becomes difficult to treat the polymer.

Thereafter, concentrated sulfuric acid is added to the above copolymerfor sulfonation, such that it contains a sulfonic acid group within therange between 0.5 and 3.0 mg equivalent/g. If the obtained sulfonationproduct contains less than 0.5 mg equivalent/g of sulfonic acid group,it cannot obtain a sufficient ion conductivity. If the content of asulfonic acid group exceeds 3.0 mg equivalent/g, a sufficient toughnesscannot be obtained, and it makes difficult to treat the sulfonate duringthe production of an electrode structure, which will be described later.

The sulfonation product of the above copolymer is then dissolved inN-methylpyrrolidone to prepare a polymer electrolyte solution.Thereafter, a membrane is formed from the polymer electrolyte solutionby the cast method followed by drying in an oven, so as to prepare, forexample, the polymer electrolyte membrane having a dry film thickness of50 μm.

The electrode structure as shown in FIG. 1 is obtained by holding thepolymer electrolyte membrane 2 between the electrode catalyst layers 1of the above electrodes followed by hot pressing. The hot pressing iscarried out, for example, at 150° C. at 2.5 MPa for 1 minute.

When a separator acting also as a gas passage is further laminated oneach of the backing layers 3, 3, the electrode structure as shown inFIG. 1 constitutes a polymer electrolyte fuel cell, which generatespower by supplying oxidizing gas to one side of the above electrodestructure and reducing gas to the other side.

In the present embodiment, in the above copolymer that is a polymerconstituting the polymer electrolyte membrane 2, when the number ofdivalent aromatic residues comprised in its main chain is denoted by X,and the number of oxy groups (—O—) comprised in its main chain isdenoted by Y, the value X/Y is within the range between 2.0 and 9.0, sothat the above polymer electrolyte membrane can obtain an excellenttoughness and an excellent ion conductivity.

Next, a method of calculating the value X/Y will be explained below.

For example, 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the firstrepeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) arepolymerized at a molar ratio of p:q:r, so as to obtain a copolymerrepresented by the following formula (7):

Herein, in the main chain of the copolymer of the above formula (7), theabove first repeating unit comprises only divalent residues of4′-(4-phenoxyphenoxy)benzophenone, and so it comprises one divalentaromatic residue and no oxy group. Moreover, the above second repeatingunit comprises six phenylene groups (—C₆H₄—) as divalent aromaticresidues and two oxy groups. Furthermore, in the main chain of thecopolymer of the above formula (7), the above third repeating unitcomprises two phenylene groups (—C₆H₄—) as divalent aromatic residuesand no oxy group.

Accordingly, in the main chain of the copolymer of the above formula(7), the number of divalent aromatic residues X can be calculated usingthe formula X=1×p+6×r+2×q, and the number of oxy groups Y can becalculated using the formula Y=2×r. As a result, the value X/Y iscalculated using the following formula (II):X/Y=(1×p+6×r+2×q)/2×r  (II)

Next, in the electrode structure in the second embodiment of the presentinvention, when the number of groups to which aromatic groups to besulfonated bind as side chains is denoted by A, the number ofnonsulfonated divalent aromatic residues is denoted by B, and the numberof oxy groups is denoted by C with respect to the total groups comprisedin the main chain of the above copolymer that is a polymer constitutingthe polymer electrolyte membrane 2, a value of the hydrophobic indexrepresented by the formula, (B/C)×(B+C)−A, is within the range of from35 to 380. Except for the above difference, the electrode structure inthe second embodiment of the present invention has a structurecompletely identical to the electrode structure in the first embodimentas shown in FIG. 1. In the present embodiment, in the above copolymerthat is a polymer constituting the polymer electrolyte membrane 2, thehydrophobic index is within the above range, so that the above polymerelectrolyte membrane can obtain an excellent oxidation resistance.

Moreover, when a separator acting also as a gas passage is furtherlaminated on each of the backing layers 3, 3, the electrode structure inthe present embodiment constitutes a polymer electrolyte fuel cell,which generates power by supplying oxidizing gas to one side of theabove electrode structure and reducing gas to the other side.

Next, a method of calculating the above hydrophobic index will beexplained below.

For example, 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the firstrepeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) arepolymerized at a molar ratio of p:q:r, so as to obtain a copolymerrepresented by the following formula (7):

Herein, sulfonation occurs only with respect to a benzene ring to whichan electron attracting group does not bind. Accordingly, in thecopolymer of the above formula (7), a sulfonic acid group is onlyintroduced into a benzene ring of the side chain of the first repeatingunit. Since the first repeating unit itself is a group in the main chainof the above copolymer, the number of groups to be sulfonated A=p.

In the copolymer of the above formula (7), a divalent aromatic residuemeans a benzene ring in each repeating unit. Accordingly, the number ofnonsulfonated divalent aromatic residues is 0 in the first repeatingunit, 2 in the third repeating unit, and 6 in the second repeating unit.Therefore, the number of nonsulfonated divalent aromatic residuesB=2q+6r. Further, in the copolymer of the above formula (7), the numberof oxy groups is 0 in the first and third repeating units, and 2 in thesecond repeating unit. Therefore, the number of oxy groups C=2r.

As a result, the hydrophobic index is calculated using the followingformula (III):(B/C)×(B+C)−A={(2q+6r)/2r}×(2q+8r)−p  (III)

Next, the present invention will be described in the following examplesand comparative examples.

EXAMPLE 1

In the present example, first,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the first repeatingunit) represented by the above formula (4), 4,4′-dichlorobenzophenone(the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 6:2:2, so as to obtain a copolymer(p:q:r=6:2:2) represented by the following formula (7):

Thereafter, concentrated sulfuric acid was added to the above copolymerfor sulfonation, so as to obtain a sulfonation product having an ionexchange capacity of 2.0 meq/g. Thereafter, the sulfonate of the abovecopolymer was dissolved in N-methylpyrrolidone to prepare a polymerelectrolyte solution. A membrane was formed from the polymer electrolytesolution by the cast method followed by drying in an oven, so as toprepare a membrane having a dry film thickness of 50 μm, and themembrane was used as the polymer electrolyte membrane 2.

Subsequently, a platinum particle was supported by carbon black (furnaceblack) at a weight ratio of carbon black:platinum=1:1, so as to preparea catalyst particle. Then, using a solution containing aperfluoroalkylene sulfonic acid polymer (e.g., Nafion (trade name) fromDuPont) as an ion conducting polymer binder, the above catalystparticles were uniformly mixed in the binder at a weight ratio ofbinder:carbon black=1:1, so as to prepare a catalyst paste.

Thereafter, carbon black was mixed with polytetrafluoroethylene (PTFE)particles at a weight ratio of carbon black:PTFE particle=4:6. Theobtained mixture was uniformly dispersed in a solvent such as ethyleneglycol to obtain a slurry. The obtained slurry was applied on the oneside of the above carbon paper followed by drying it, so as to obtain asubstrate layer. Then, two of the backing layers 3 were prepared, eachof which consisted of the substrate layer and the carbon paper.

Thereafter, the above catalyst paste was screen printed on each of theabove backing layers 3, so that 0.5 mg/cm2 platinum was kept thereon.Then, drying was carried out so as to prepare an electrode catalystlayer 1. Thus, a pair of electrodes were prepared, each of whichconsisted of the electrode catalyst layer 1 and the backing layer 3.

Thereafter, the polymer electrolyte membrane 2 was held between theelectrode catalyst layers 1 of the above electrodes, and they were hotpressed to form the electrode structure as shown in FIG. 1.

In the present example, since p:q:r=6:2:2, X=22 and Y=4. Accordingly,X/Y=5.5 according to the above formula (II).

Subsequently, regarding the electrode structure in the present example,the toughness and ion conductivity of the polymer electrolyte membrane2, and the power generation efficiency of the electrode structure wereevaluated.

The polymer electrolyte membrane 2 was processed in a dumbbell rated toJIS 7, and the tensile elongation at break was measured under theconditions of a distance between chucks of 20 mm, a crosshead speed of50 mm/min, a temperature of 25° C. and a relative humidity of 50%. Theobtained tensile elongation at break was defined as toughness. Theresults are shown in Table 1. The relationship between the value X/Y andthe toughness (tensile elongation at break) is shown in FIG. 2.

Regarding the ion conductivity, the polymer electrolyte membrane 2 washeld between two platinum electrodes, and the ion conductivity of themembrane was then measured by the alternating two-terminal method(frequency: 10 kHz) under the conditions of a temperature of 85° C. anda relative humidity of 90%. The results are shown in Table 1. Therelationship between the value X/Y and the ion conductivity is shown inFIG. 3.

The power generation efficiency was evaluated as follows. The aboveelectrode structure was used for a single cell. Air was supplied to onebacking layer 3 as an oxygen electrode, whereas pure hydrogen wassupplied to the other backing layer 3 as a fuel electrode, so as togenerate electric power. Power generation conditions were a temperatureof 90° C., a relative humidity of 50% on the fuel electrode side, and arelative humidity of 80% on the oxygen electrode side. The cell voltagewas measured at a current density of 0.5 A/cm². If the measured cellvoltage was 0.4 V or greater, it was evaluated that the cell had a goodpower generation efficiency. The results are shown in Table 1.

EXAMPLE 2

In the present example, the electrode structure as shown in FIG. 1 wasobtained completely in the same manner as in Example 1 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the firstrepeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 6:3:1, so as to obtain a copolymer (pq:r=6:3:1) represented by the above formula (7).

In the present example, since p:q:r=6:3:1, X=18 and Y=2. Accordingly,X/Y=9.0 according to the above formula (II).

Subsequently, regarding the electrode structure in the present example,the toughness, the ion conductivity, and the power generation efficiencywere evaluated in the same manner as in Example 1. The results are shownin Table 1 and FIGS. 2 and 3.

EXAMPLE 3

In the present example, the electrode structure as shown in FIG. 1 wasobtained completely in the same manner as in Example 1 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the firstrepeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 6:1:3, so as to obtain a copolymer(p:q:r=6:1:3) represented by the above formula (7).

In the present example, since p:q:r=6:1:3, X=26 and Y=6. Accordingly,X/Y=4.3 according to the above formula (II).

Subsequently, regarding the electrode structure in the present example,the toughness, the ion conductivity, and the power generation efficiencywere evaluated in the same manner as in Example 1. The results are shownin Table 1 and FIGS. 2 and 3.

EXAMPLE 4

In the present example, 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone(the first repeating unit) represented by the above formula (4) waspolymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) at amolar ratio of 5:5, while not using 4,4′-dichlorobenzophenone (the thirdrepeating unit) at all, so as to obtain a copolymer (p:r=5:5)represented by formula (8) indicated below. Then, the sulfonationproduct of the obtained copolymer was used as the polymer electrolytemembrane 2. Except for the above difference, the electrode structure asshown in FIG. 1 was obtained completely in the same manner as in Example1.

The copolymer of formula (8) corresponds to the case of q=0 in the abovecopolymer of formula (7). Accordingly, X, Y, and X/Y can be calculatedin the same manner as in Example 1. In the present example, sincep:r=5:5 and q=0, X=35 and Y=10. Accordingly, X/Y=3.5 according to theabove formula (II).

Subsequently, regarding the electrode structure in the present example,the toughness, the ion conductivity, and the power generation efficiencywere evaluated in the same manner as in Example 1. The results are shownin Table 1 and FIGS. 2 and 3.

EXAMPLE 5

In the present example, 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone(the first repeating unit) represented by the above formula (4) waspolymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) at amolar ratio of 9:1, while not using 4,4′-dichlorobenzophenone (the thirdrepeating unit) at all, so as to obtain a copolymer (p:r=9:1)represented by the above formula (8). Then, the sulfonation product ofthe obtained copolymer was used as the polymer electrolyte membrane 2.Except for the above difference, the electrode structure as shown inFIG. 1 was obtained completely in the same manner as in Example 1.

In the present example, since p:r=9:1 and q=0, X=15 and Y=2.Accordingly, X/Y=7.5 according to the above formula (II).

Subsequently, regarding the electrode structure in the present example,the toughness, the ion conductivity, and the power generation efficiencywere evaluated in the same manner as in Example 1. The results are shownin Table 1 and FIGS. 2 and 3.

EXAMPLE 6

In the present example, 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone(the first repeating unit) represented by the above formula (4) waspolymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) at amolar ratio of 9:15, while not using 4,4′-dichlorobenzophenone (thethird repeating unit) at all, so as to obtain a copolymer (p:r=9:15)represented by the above formula (8). Then, the sulfonation product ofthe obtained copolymer was used as the polymer electrolyte membrane 2.Except for the above difference, the electrode structure as shown inFIG. 1 was obtained completely in the same manner as in Example 1.

In the present example, since p:r=9:15 and q=0, X=99 and Y=30.Accordingly, X/Y=3.3 according to the above formula (II).

Subsequently, regarding the electrode structure in the present example,the toughness, the ion conductivity, and the power generation efficiencywere evaluated in the same manner as in Example 1. The results are shownin Table 1 and FIGS. 2 and 3.

EXAMPLE 7

In the present example, the electrode structure as shown in FIG. 1 wasobtained completely in the same manner as in Example 1 with theexception that a polyether copolymer represented by formula (9)indicated below was used instead of the copolymer represented by theabove formula (7), and that a sulfonation product having an ion exchangecapacity of 1.5 meq/g obtained by adding concentrated sulfuric acid tothe polyether copolymer for sulfonation was used as the polymerelectrolyte membrane 2.

The main chain of the polyether copolymer of the above formula (9)contains four phenylene groups as divalent aromatic residues and two oxygroups. Accordingly, in the present example, X=4, Y=2, and X/Y=2.0.

Subsequently, regarding the electrode structure in the present example,the toughness, the ion conductivity, and the power generation efficiencywere evaluated in the same manner as in Example 1. The results are shownin Table 1 and FIGS. 2 and 3.

COMPARATIVE EXAMPLE 1

In the present comparative example, the electrode structure as shown inFIG. 1 was obtained completely in the same manner as in Example 1 withthe exception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (thefirst repeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 5:4:1, so as to obtain a copolymer(p:q:r=5:4:1) represented by the above formula (7).

In the present comparative example, since p:q r=5:4:1, X=19 and Y=2.Accordingly, X/Y=9.5 according to the above formula (II).

Subsequently, regarding the electrode structure in the presentcomparative example, the toughness, the ion conductivity, and the powergeneration efficiency were evaluated in the same manner as in Example 1.The results are shown in Table 1 and FIGS. 2 and 3.

COMPARATIVE EXAMPLE 2

In the present comparative example, the electrode structure as shown inFIG. 1 was obtained completely in the same manner as in Example 1 withthe exception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (thefirst repeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 4:5:1, so as to obtain a copolymer(p:q:r=4:5:1) represented by the above formula (7).

In the present comparative example, since p:q:r=4:5:1, X=20 and Y=2.Accordingly, X/Y=10.0 according to the above formula (II).

Subsequently, regarding the electrode structure in the presentcomparative example, the toughness, the ion conductivity, and the powergeneration efficiency were evaluated in the same manner as in Example 1.The results are shown in Table 1 and FIGS. 2 and 3.

COMPARATIVE EXAMPLE 3

In the present comparative example, the electrode structure as shown inFIG. 1 was obtained completely in the same manner as in Example 1 withthe exception that polyether ether ketone represented by formula (10)indicated below was used instead of the copolymer represented by theabove formula (7), and that a sulfonate having an ion exchange capacityof 1.5 meq/g was obtained by adding concentrated sulfuric acid to thepolyether ether ketone for sulfonation and it was used as the polymerelectrolyte membrane 2.

The main chain of the polyether ether ketone of the above formula (10)contains three phenylene groups as divalent aromatic residues and twooxy groups. Accordingly, in the present comparative example, X=3, Y=2,and X/Y=1.5.

Subsequently, regarding the electrode structure in the presentcomparative example, the toughness, the ion conductivity, and the powergeneration efficiency were evaluated in the same manner as in Example 1.The results are shown in Table 1 and FIGS. 2 and 3.

TABLE 1 Tensile Ion Power elongation at conductivity generation X/Ybreak (%) (S/cm) efficiency Example 1 5.5 27 0.14 G Example 2 9.0 18 0.1G Example 3 4.3 27 0.13 G Example 4 3.5 28 0.12 G Example 5 7.5 23 0.12G Example 6 3.3 27 0.12 G Example 7 2.0 30 0.08 G Comparative 9.5 10 0.1G Example 1 Comparative 10.0 5 0.08 G Example 2 Comparative 1.5 30 0.045P Example 3 Power generation efficiency: G . . . A cell voltage of 0.4 Vor greater at a current density of 5 A/cm² P . . . A cell voltage ofless than 0.4 V at a current density of 5 A/cm²

In the electrode structures of Examples 1 to 7, the value X/Y that isthe ratio between the number X of divalent aromatic residues comprisedin the main chain of the polymer forming the polymer electrolytemembrane 2 and the number Y of oxy groups comprised in the same abovemain chain is within the range between 2.0 and 9.0. As is clear from theresults shown in Table 1 and FIGS. 2 and 3, these electrode structuresall comprise the polymer electrolyte membrane 2 that is excellent intoughness (tensile elongation at break) and ion conductivity, andfurther, they have a good power generation efficiency.

In contrast, the electrode structures of Comparative Examples 1 and 2 inwhich the above X/Y in the main chain of the polymer constituting thepolymer electrolyte membrane 2 exceeds 9.0 are excellent in the ionconductivity of their polymer electrolyte membrane 2, but they areclearly poor in the toughness of the same above membrane 2. In addition,the electrode structure of Comparative Example 3 in which the above X/Yin the main chain of the polymer constituting the polymer electrolytemembrane 2 is less than 2.0 is excellent in the toughness of themembrane 2, but it is clearly poor in the ion conductivity of themembrane and its power generation efficiency is also insufficient.

EXAMPLE 8

In the present example, first,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the first repeatingunit) represented by the above formula (4), 4,4′-dichlorobenzophenone(the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 6:2:2, so as to obtain a copolymer(p:q:r=6:2:2) represented by the following formula (7):

In the present example, the hydrophobic index of the above copolymer wascalculated using the above formula (III), and it holds that(B/C)×(B+C)−A=(16/4)×(16+4)−6=74.

Thereafter, concentrated sulfuric acid was added to the above copolymerfor sulfonation, so as to obtain a sulfonate having an ion exchangecapacity of 2.0 meq/g. Thereafter, the sulfonation product of the abovecopolymer was dissolved in N-methylpyrrolidone to prepare a polymerelectrolyte solution. A membrane was formed from the polymer electrolytesolution by casting, followed by drying in an oven, so as to prepare amembrane having a dry film thickness of 50 μm, and the membrane was usedas the polymer electrolyte membrane 2.

Subsequently, a platinum particle was supported by carbon black (furnaceblack) at a weight ratio of carbon black:platinum=1:1, so as to preparea catalyst particle. Then, using a solution containing aperfluoroalkylene sulfonic acid polymer (e.g., Nafion (trade name) fromDuPont) as an ion conducting polymer binder, the above catalystparticles were uniformly dispersed in the binder at a weight ratio ofbinder:carbon black=1:1, so as to prepare a catalyst paste.

Thereafter, carbon black was mixed with polytetrafluoroethylene (PTFE)particles at a weight ratio of carbon black:PTFE particle=4:6. Theobtained mixture was uniformly dispersed in a solvent such as ethyleneglycol to obtain a slurry. The obtained slurry was applied on the oneside of the above carbon paper followed by drying it, so as to obtain asubstrate layer. Then, two of the backing layers 3 were prepared, eachof which consisted of the substrate layer and carbon paper.

Thereafter, the above catalyst paste was screen printed on each of theabove backing layers 3, so that 0.5 mg/cm² platinum was kept thereon.Then, drying was carried out so as to prepare an electrode catalystlayer 1. Thus, a pair of electrodes were prepared, each of whichconsisted of the electrode catalyst layer 1 and the backing layer 3.

Thereafter, the polymer electrolyte membrane 2 was held between theelectrode catalyst layers 1 of the above electrodes, and they were thenhot pressed to obtain the electrode structure as shown in FIG. 1.

Subsequently, regarding the electrode structure in the present example,the oxidation resistance of the polymer electrolyte membrane 2 and thepower generation efficiency of the electrode structure were evaluated.

The oxidation resistance of the polymer electrolyte membrane 2 wasmeasured as follows. The polymer electrolyte membrane 2 was immersed for9 hours in a 40° C. aqueous solution (Fenton's reagent) containing 3%H2O2 and Fe with a concentration of 20 ppm, and then its weightreduction rate (%) was measured. The oxidation resistance was defined assuch a weight reduction rate. The above weight reduction rate indicatesthe amount of the polymer electrolyte membrane 2 dissolved in the abovereagent. The smaller the figure, the higher the oxidation resistance.The results are shown in Table 2. In addition, the relationship betweenthe hydrophobic index and the oxidation resistance (weight reductionrate) is shown in FIG. 4.

The power generation efficiency was evaluated as follows. The aboveelectrode structure was used for a single cell. The evaluation wascarried out by supplying air to one backing layer 3 as an oxygenelectrode and pure hydrogen to the other backing layer 3 as a fuelelectrode, so as to generate power. Power generation conditions were atemperature of 90° C., a relative humidity of 50% on the fuel electrodeside, and a relative humidity of 80% on the oxygen electrode side. Thecell voltage was measured at a current density of 0.5 A/cm². If themeasured cell voltage was 0.4 V or greater, it was evaluated that thecell had a good power generation efficiency. The results are shown inTable 2.

EXAMPLE 9

In the present example, the electrode structure as shown in FIG. 1 wasobtained completely in the same manner as in Example 8 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the firstrepeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 6:3:1, so as to obtain a copolymer(p:q:r=6:3:1) represented by the above formula (7).

In the present example, since p:q:r=6:3:1, the hydrophobic index of theabove copolymer was calculated using the above formula (III), and itholds that (B/C)×(B+C)−A=78.

Subsequently, regarding the electrode structure in the present example,the oxidation resistance of the polymer electrolyte membrane 2 and thepower generation efficiency of the electrode structure were evaluated inthe same manner as in Example 8. The results are shown in Table 2 andFIG. 4.

EXAMPLE 10

In the present example, the electrode structure as shown in FIG. 1 wasobtained completely in the same manner as in Example 8 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the firstrepeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 6:1:3, so as to obtain a copolymer(p:q:r=6:1:3) represented by the above formula (7).

In the present example, since p:q:r=6:1:3, the hydrophobic index of theabove copolymer was calculated using the above formula (III), and itholds that (B/C)×(B+C)−A=80.

Subsequently, regarding the electrode structure in the present example,the oxidation resistance of the polymer electrolyte membrane 2 and thepower generation efficiency of the electrode structure were evaluated inthe same manner as in Example 8. The results are shown in Table 2 andFIG. 4.

EXAMPLE 11

In the present example, the electrode structure as shown in FIG. 1 wasobtained completely in the same manner as in Example 8 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the firstrepeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 5:4:1, so as to obtain a copolymer(p:q:r=5:4:1) represented by the above formula (7).

In the present example, since p:q:r=5:4:1, the hydrophobic index of theabove copolymer was calculated using the above formula (III), and itholds that (B/C)×(B+C)−A=107.

Subsequently, regarding the electrode structure in the present example,the oxidation resistance of the polymer electrolyte membrane 2 and thepower generation efficiency of the electrode structure were evaluated inthe same manner as in Example 8. The results are shown in Table 2 andFIG. 4.

EXAMPLE 12

In the present example, the electrode structure as shown in FIG. 1 wasobtained completely in the same manner as in Example 8 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the firstrepeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 4:5:1, so as to obtain a copolymer(p:q:r=4:5:1) represented by the above formula (7).

In the present example, since p:q:r=4:5:1, the hydrophobic index of theabove copolymer was calculated using the above formula (III), and itholds that (B/C)×(B+C)−A=140.

Subsequently, regarding the electrode structure in the present example,the oxidation resistance of the polymer electrolyte membrane 2 and thepower generation efficiency of the electrode structure were evaluated inthe same manner as in Example 8. The results are shown in Table 2 andFIG. 4.

EXAMPLE 13

In the present example, the electrode structure as shown in FIG. 1 wasobtained completely in the same manner as in Example 8 with theexception that 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the firstrepeating unit) represented by the above formula (4),4,4′-dichlorobenzophenone (the third repeating unit), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 3:1:1, so as to obtain a copolymer(p:q:r=3:1:1) represented by the above formula (7).

In the present example, since p:q:r=3:1:1, the hydrophobic index of theabove copolymer was calculated using the above formula (III), and itholds that (B/C)×(B+C)−A=37.

Subsequently, regarding the electrode structure in the present example,the oxidation resistance of the polymer electrolyte membrane 2 and thepower generation efficiency of the electrode structure were evaluated inthe same manner as in Example 8. The results are shown in Table 2 andFIG. 4.

EXAMPLE 14

In the present example, 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone(the first repeating unit) represented by the above formula (4) waspolymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) at amolar ratio of 9:8, so as to obtain a copolymer (p:r=9:8) represented byformula (8) indicated below. The thus obtained copolymer was usedinstead of the copolymer of the above formula (7). Concentrated sulfuricacid was added to the copolymer of formula (8) for sulfonation, so as toobtain a sulfonation product having an ion exchange capacity of 1.9meq/g. Except for the above differences, the electrode structure asshown in FIG. 1 was obtained completely in the same manner as in Example8.

The copolymer of formula (8) corresponds to the case of q=0 in the abovecopolymer of formula (7). In the present example, since p:r=9:8, thehydrophobic index of the above copolymer was calculated using the aboveformula (III), and it holds that (B/C)×(B+C)−=183.

Subsequently, regarding the electrode structure in the present example,the oxidation resistance of the polymer electrolyte membrane 2 and thepower generation efficiency of the electrode structure were evaluated inthe same manner as in Example 8. The results are shown in Table 2 andFIG. 4.

EXAMPLE 15

In the present example, 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone(the first repeating unit) represented by the above formula (4) waspolymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) at amolar ratio of 9:12, so as to obtain a copolymer (p:r=9:12) representedby the above formula (8). The thus obtained copolymer was used insteadof the copolymer of the above formula (7). Concentrated sulfuric acidwas added to the copolymer of formula (8) for sulfonation, so as toobtain a sulfonation product having an ion exchange capacity of 2.0meq/g. Except for the above differences, the electrode structure asshown in FIG. 1 was obtained completely in the same manner as in Example8.

The copolymer of formula (8) corresponds to the case of q=0 in the abovecopolymer of formula (7). In the present example, since p:r=9:12, thehydrophobic index of the above copolymer was calculated using the aboveformula (III), and it holds that (B/C)×(B+C)−A=279.

Subsequently, regarding the electrode structure in the present example,the oxidation resistance of the polymer electrolyte membrane 2 and thepower generation efficiency of the electrode structure were evaluated inthe same manner as in Example 8. The results are shown in Table 2 andFIG. 4.

EXAMPLE 16

In the present example, 2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone(the first repeating unit) represented by the above formula (4) waspolymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) at amolar ratio of 9:15, so as to obtain a copolymer (p:r=9:15) representedby the above formula (8). The thus obtained copolymer was used insteadof the copolymer of the above formula (7). Concentrated sulfuric acidwas added to the copolymer of formula (8) for sulfonation, so as toobtain a sulfonation product having an ion exchange capacity of 2.0meq/g. Except for the above differences, the electrode structure asshown in FIG. 1 was obtained completely in the same manner as in Example8.

The copolymer of formula (8) corresponds to the case of q=0 in the abovecopolymer of formula (7). In the present example, since p:r=9:15, thehydrophobic index of the above copolymer was calculated using the aboveformula (III), and it holds that (B/C)×(B+C)−A=351.

Subsequently, regarding the electrode structure in the present example,the oxidation resistance of the polymer electrolyte membrane 2 and thepower generation efficiency of the electrode structure were evaluated inthe same manner as in Example 8. The results are shown in Table 2 andFIG. 4.

COMPARATIVE EXAMPLE 4

In the present comparative example, the electrode structure as shown inFIG. 1 was obtained completely in the same manner as in Example 8 withthe exception that polyether ether ketone represented by formula (10)indicated below was used instead of the copolymer represented by theabove formula (7), and that a sulfonation product having an ion exchangecapacity of 1.5 meq/g was obtained by adding concentrated sulfuric acidto the above polyether ether ketone for sulfonation.

In the polyether ether ketone represented by the above formula (10),only the benzene ring present between two oxy groups that are electronreleasing groups is sulfonated, but other benzene rings binding toketone groups that are electron attracting groups are therefore notsulfonated. Thus, in the present comparative example, the number ofgroups to be sulfonated A=1, the number of non-sulfonated divalentaromatic residues B=2, and the number of oxy groups C=2. Accordingly,when the hydrophobic index was calculated using the above formula (I),it holds that (B/C)×(B+C)−A=(2/2)×(2+2)−1=3.

Subsequently, regarding the electrode structure in the presentcomparative example, the oxidation resistance of the polymer electrolytemembrane 2 and the power generation efficiency of the electrodestructure were evaluated in the same manner as in Example 8. The resultsare shown in Table 2 and FIG. 4.

COMPARATIVE EXAMPLE 5

In the present comparative example, the electrode structure as shown inFIG. 1 was obtained completely in the same manner as in Example 8 withthe exception that a polyether ether ketone copolymer represented byformula (9) indicated below was used instead of the copolymerrepresented by the above formula (7), and that a sulfonation producthaving an ion exchange capacity of 1.5 meq/g was obtained by addingconcentrated sulfuric acid to the above polyether ether ketone copolymerfor sulfonation.

In the polyether ether ketone copolymer represented by formula (9)indicated below, only the benzene rings of a fluorene residuerepresented by formula (11) indicated below are sulfonated, but otherbenzene rings are not sulfonated.

Due to steric hindrance, the above sulfonation easily occurs on thebenzene rings of the side chain, but hardly occurs on any benzene ringof the main chain. As a result, although the polyether ether ketonecopolymer represented by the above formula (9) comprises benzene ringsbinding to oxy groups that are electron releasing groups and methylenegroups (which make up a part of the above fluorene residue) in the mainchain thereof, the benzene rings are not sulfonated.

Thus, in the present comparative example, the number of groups to besulfonated A=1, the number of non-sulfonated divalent aromatic residuesB=4, and the number of oxy groups C=2. Accordingly, when the hydrophobicindex was calculated using the above formula (I), it holds that(B/C)×(B+C)−A=(2/2)×(2+2)−1=11.

Subsequently, regarding the electrode structure in the presentcomparative example, the oxidation resistance of the polymer electrolytemembrane 2 and the power generation efficiency of the electrodestructure were evaluated in the same manner as in Example 8. The resultsare shown in Table 2 and FIG. 4.

COMPARATIVE EXAMPLE 6

In the present comparative example,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the first repeatingunit) represented by the above formula (4) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) at amolar ratio of 9:20, so as to obtain a copolymer (p:r=9:20) representedby the above formula (8). The thus obtained copolymer was used insteadof the copolymer of the above formula (7). Concentrated sulfuric acidwas added to the copolymer of the above formula (8) for sulfonation, soas to obtain a sulfonation product having an ion exchange capacity of1.9 meq/g. Except for the above differences, the electrode structure asshown in FIG. 1 was obtained completely in the same manner as in Example8.

The copolymer of formula (8) corresponds to the case of q=0 in the abovecopolymer of formula (7). In the present comparative example, sincep:r=9:20, the hydrophobic index of the above copolymer was calculatedusing the above formula (III), and it holds that (B/C)×(B+C)−A=471.

Subsequently, regarding the electrode structure in the presentcomparative example, the oxidation resistance of the polymer electrolytemembrane 2 and the power generation efficiency of the electrodestructure were evaluated in the same manner as in Example 8. The resultsare shown in Table 2 and FIG. 4.

COMPARATIVE EXAMPLE 7

In the present comparative example,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the first repeatingunit) represented by the above formula (4) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) at amolar ratio of 1:1, so as to obtain a copolymer (p:r=1:1) represented bythe above formula (8). The thus obtained copolymer was used instead ofthe copolymer of the above formula (7). Concentrated sulfuric acid wasadded to the copolymer of the above formula (8) for sulfonation, so asto obtain a sulfonation product having an ion exchange capacity of 1.9meq/g. Except for the above differences, the electrode structure asshown in FIG. 1 was obtained completely in the same manner as in Example8.

The copolymer of formula (8) corresponds to the case of q=0 in the abovecopolymer of formula (7). In the present comparative example, sincep:r=1:1, the hydrophobic index of the above copolymer was calculatedusing the above formula (III), and it holds that (B/C)×(B+C)−A=23.

Subsequently, regarding the electrode structure in the presentcomparative example, the oxidation resistance of the polymer electrolytemembrane 2 and the power generation efficiency of the electrodestructure were evaluated in the same manner as in Example 8. The resultsare shown in Table 2 and FIG. 4.

COMPARATIVE EXAMPLE 8

In the present comparative example,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the first repeatingunit) represented by the above formula (4) was polymerized with2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) at amolar ratio of 9:1, so as to obtain a copolymer (p:r=9:1) represented bythe above formula (8). The thus obtained copolymer was used instead ofthe copolymer of the above formula (7). Concentrated sulfuric acid wasadded to the copolymer of the above formula (8) for sulfonation, so asto obtain a sulfonation product having an ion exchange capacity of 1.9meq/g. Except for the above differences, the electrode structure asshown in FIG. 1 was obtained completely in the same manner as in Example8.

The copolymer of formula (8) corresponds to the case of q=0 in the abovecopolymer of formula (7). In the present comparative example, sincep:r=9:1, the hydrophobic index of the above copolymer was calculatedusing the above formula (III), and it holds that (B/C)×(B+C)−A=15.

Subsequently, regarding the electrode structure in the presentcomparative example, the oxidation resistance of the polymer electrolytemembrane 2 and the power generation efficiency of the electrodestructure were evaluated in the same manner as in Example 8. The resultsare shown in Table 2 and FIG. 4.

COMPARATIVE EXAMPLE 9

In the present comparative example,2,5-dichloro-4′-(4-phenoxyphenoxy)benzophenone (the first repeatingunit) represented by the above formula (4), polyether ether ketonerepresented by the above formula (9), and2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane(the second repeating unit) represented by the above formula (5) werepolymerized at a molar ratio of 6:2:1, so as to obtain a copolymerrepresented by formula (12) indicated below. The thus obtained copolymerwas used instead of the copolymer of the above formula (7). Concentratedsulfuric acid was added to the copolymer of the above formula (12) forsulfonation, so as to obtain a sulfonation product having an ionexchange capacity of 2.0 meq/g. Except for the above differences, theelectrode structure as shown in FIG. 1 was obtained completely in thesame manner as in Example 8.

In the copolymer represented by the above formula (12), sulfonationoccurs only on the benzene rings of the side chain of the firstrepeating unit. The third repeating unit also comprises a benzene ringthat intervenes between two oxy groups that are electron releasinggroups. However, as described above, since sulfonation easily occurs onthe benzene rings of the side chain, but hardly occurs on any benzenering of the main chain due to steric hindrance, the benzene ring of thethird repeating unit is not sulfonated.

Thus, in the present comparative example, the number of groups to besulfonated A=1×6=6, the number of nonsulfonated divalent aromaticresidues B=3×2+6×1=12, and the number of oxy groups C=2×2+2×1=6.Accordingly, when the hydrophobic index was calculated using the aboveformula (I), it holds that (B/C)×(B+C)−A=(12/6)×(12+6)−6=30.

Subsequently, regarding the electrode structure in the presentcomparative example, the oxidation resistance of the polymer electrolytemembrane 2 and the power generation efficiency of the electrodestructure were evaluated in the same manner as in Example 8. The resultsare shown in Table 2 and FIG. 4.

TABLE 2 Weight Power Hydrophobic reduction rate generation A B C index(%) efficiency Example 8 6 16 4 74 16 G Example 9 6 12 2 78 15 G Example10 6 20 6 80 15 G Example 11 5 14 2 107 14 G Example 12 4 16 2 140 12 GExample 13 3 8 2 37 20 G Example 14 9 48 16 183 10 G Example 15 9 72 24279 10 G Example 16 9 90 30 351 18 G Comparative 1 2 2 3 65 G Example 4Comparative 1 4 2 11 65 G Example 5 Comparative 9 120 40 471 30 PExample 6 Comparative 1 6 2 23 30 G Example 7 Comparative 9 6 2 15 40 GExample 8 Comparative 6 12 6 30 25 G Example 9 Power generationefficiency: G . . . A cell voltage of 0.4 V or greater at a currentdensity of 5 A/cm² P . . . A cell voltage of less than 0.4 V at acurrent density of 5 A/cm²

Table 2 and FIG. 4 clearly show that Examples 8 to 16 in which thehydrophobic index is within the range between 35 to 380 have a smallweight reduction rate resulting in an excellent oxidation resistance,and an excellent power generation efficiency. In contrast, bothComparative Examples 4, 5, and 7 to 9 in which the hydrophobic index isless than 35 and Comparative Example 6 in which the hydrophobic indexexceeds 380 have a large weight reduction rate, and therefore theycannot obtain a sufficient oxidation resistance. Moreover, they are poorin power generation efficiency.

When the hydrophobic index exceeds 380 as in the case of ComparativeExample 6, if the amount of repeating units comprising non-sulfonateddivalent aromatic residues is excessive to the amount of repeating unitscomprising groups to be sulfonated, the length of a main chainlengthens. As a result, it is considered that association oragglutination of molecules takes place actively, and that oxidationresistance decreases.

INDUSTRIAL APPLICABILITY

The present invention can be used for a polymer electrolyte fuel cell,which is used in vehicles and the like.

1. An electrode structure for a polymer electrolyte fuel cell,comprising: a pair of electrode catalyst layers, and a polymerelectrolyte membrane held between the electrode catalyst layers, saidpolymer electrolyte membrane comprising a sulfonation product of apolymer, wherein said polymer comprises a main chain having a pluralityof divalent aromatic residues bound to one another directly or throughoxy groups or divalent groups other than aromatic residues, and sidechains comprising sulfonated aromatic groups, and wherein the number ofdivalent aromatic residues comprised in the main chain of said polymeris denoted by X, the number of oxy groups comprised in the main chain ofsaid polymer is denoted by Y, and the value X/Y is within the range offrom 2.0 to 9.0.
 2. The electrode structure for a polymer electrolytefuel cell according to claim 1, wherein the number of sulfonatedaromatic groups is denoted by A, the number of non-sulfonated divalentaromatic residues is denoted by B, the number of oxy groups is denotedby C with respect to the total groups comprised in the main chain ofsaid polymer, and the value (B/C)×(B+C)−A is within the range of from 35to
 380. 3. The electrode structure for a polymer electrolyte fuel cellaccording to anyone of claims 1 and 2, wherein the main chain of saidpolymer comprises a first repeating unit represented by the followinggeneral formula (1) and a second repeating unit represented by thefollowing general formula (2):

wherein A represents an electron attracting group, B represents anelectron releasing group, n is an integer of 0 or 1, and a benzene ringincludes benzene and derivatives thereof, and

wherein A represents an electron attracting group, Y represents—C(CF₃)₂— or —SO₂—, and a benzene ring includes benzene and derivativesthereof.
 4. The electrode structure for a polymer electrolyte fuel cellaccording to anyone of claims 1 and 2, wherein the main chain of saidpolymer comprises a first repeating unit represented by the followinggeneral formula (1), a second repeating unit represented by thefollowing general formula (2), and a third repeating unit represented bythe following general formula (3):

wherein A represents an electron attracting group, B represents anelectron releasing group, n is an integer of 0 or 1, and a benzene ringincludes benzene and derivatives thereof,

wherein A represents an electron attracting group, Y represents—C(CF₃)₂— or —SO₂—, and a benzene ring includes benzene and derivativesthereof, and

wherein B is an electron releasing group.
 5. A polymer electrolyte fuelcell comprising: an electrode structure comprising a pair of electrodecatalyst layers and a polymer electrolyte membrane held between theelectrode catalyst layers, said polymer electrolyte membrane comprisinga polymer comprising a main chain, in which a plurality of divalentaromatic residues are bound to one another directly or through oxygroups or divalent groups other than aromatic residues, and side chainscomprising sulfonated aromatic groups, wherein the number of divalentaromatic residues comprised in the main chain of said polymer is denotedby X, the number of oxy groups comprised in the main chain of saidpolymer is denoted by Y, and the value X/Y is within the range of from2.0 to 9.0, and wherein electric power is generated when an oxidizinggas is supplied to one side of said electrode structure and a reducinggas is supplied to another side of said electrode structure.
 6. Thepolymer electrolyte fuel cell according to claim 5, wherein the numberof sulfonated aromatic groups is denoted as A, the number ofnon-sulfonated divalent aromatic residues is denoted as B, the number ofoxy groups is denoted as C with respect to the total groups comprised inthe main chain of said polymer, and the value (B/C)×(B+C)−A is withinthe range of from 35 to
 380. 7. The polymer electrolyte fuel cellaccording to claim 5, wherein the number of sulfonated aromatic groupsis denoted as A, the number of nonsulfonated divalent aromatic residuesis denoted as B, the number of oxy groups is denoted as C with respectto the total groups comprised in the main chain of said polymer, and thevalue (B/C)×(B+C)−A is within the range of from 35 to 380.